Huge Drop in Google AdSense Revenue This Month?

There are many reporting huge drops in the average revenue they have been earning this month from Google AdSense. A DigitalPoint Forums thread has several AdSense publishers complaining in the decline in revenues.

Some see $30 per day reductions, some see $20 reductions, some are seeing worse.

Is it smart pricing at its best or just overall revenue share declines? Or maybe it is something about the quality of those sites in the AdSense network?